Wireless communication devices or units such as communication units and base transceivers providing data and voice services for users operating in corresponding systems are known. As these systems have evolved more sophisticated encoding and modulation schemes are being employed. Present systems often rely at least in part on schemes where orthogonality between signals is utilized to distinguish a signal from all others. Classic examples of such a systems are those employing Orthogonal Frequency Division Multiplexing (OFDM) modulation and protocols, such as those often used to provide Digital Subscriber Line (DSL) services, wireless local area network services using IEEE 802.11a protocol, and the like.
OFDM based protocols are often employed due to the possibility of high transmission efficiencies usually assessed in information bits per second per hertz terms. These systems can be modeled as a plurality of carriers (sub-carriers) that are closely spaced in frequency with each carrier being modulated by a unique frequency offset value. Also, the relatively long symbol period compared to the channel delay spread mitigates frequency selective fading effects. These sub-carriers as modulated can be closely spaced since they are orthogonal and thus any interference can be negated at a target receiver. However, when these individual carriers as modulated are not orthogonal the inter-carrier interference effects cannot be negated and the available bit error rate increases dramatically. It has been shown that the orthogonality between the plurality of sub-carriers is strongly dependent on knowing with precision the frequency that was used to transmit the OFDM signals.
Wireless receivers and transmitters essentially never have the same frequency reference. Furthermore a wireless channel can contribute to differences between a transmitted and received frequency due for example to relative motion between the transmitter and receiver or reflectors impacting the wireless channel. Thus the receiver must determine or estimate the frequency offset between the receiver frequency and the OFDM signal that is being received. While techniques exist for making this estimate, they suffer from various problems, such as limited accuracy or excess complexity or delay in providing the estimate. It is almost always advantageous, particularly in a wireless receiver to use less computational complexity and incur minimal delay, especially in bursty information transfers. A need exists for improved methods and apparatus for estimating frequency offset for an OFDM signal.